Reduce false sharing between pathInfoCache and Store

`perf c2c` shows a lot of cacheline conflicts between purely read-only
Store methods (like `parseStorePath()`) and the Sync classes. So
allocate pathInfoCache separately to avoid that.

src/libutil/include/nix/util/ref.hh

@@ -18,6 +18,9 @@ private:
     std::shared_ptr<T> p;
 
 public:
+
+    using element_type = T;
+
     explicit ref(const std::shared_ptr<T> & p)
         : p(p)
     {

src/libutil/include/nix/util/sync.hh

@@ -36,6 +36,8 @@ private:
 
 public:
 
+    using element_type = T;
+
     SyncBase() {}
 
     SyncBase(const T & data)